The present invention relates generally to fluid sampling systems either employing an analyzer selectively coupled to a sample location of a fluid for extracting a fluid sample therefrom, or alternatively for collection of a sample for subsequent analysis by an analyzer at a remote location.
1. Technical Field
Most gas and liquid chemical composition and physical property analyzers need to extract fluid samples, either continuously or intermittently, from the location where the fluid to be tested resides (the “sample location”—Si in the Figures). This sample fluid is then transported to the inside of the analyzer detector to obtain a desired test result or to a storage vessel from which sampling later occurs using another extractive sampling system. For the purposes of this disclosure, either the detector or the storage vessel constitutes a “sample destination”.
2. Description of Prior Art
A variety of extractive sampling systems exist. The simplest consist of a fluid transport tube of varying diameters and lengths with or without an in-line sample pump that moves the fluid from the sample location to the sample destination. Simple fluid sampling systems are reliable and allow accurate fluid properties analysis only if the fluid is very clean, the fluid has physical and chemical properties that do not change during transport in the sampling system, the sampling is not continuous, and no valves or other flow control components are present in the flow path. Most useful sampling system applications require additional components or they will plug in a short time or damage the analyzer or flow control components.
If the fluid contains suspended solid matter, and the solid material either obstructs the flow of the fluid or interferes in some way with the pump, connecting valve, or the analyzer detector, a particle filter is often installed in or near the sample location end of the fluid transport tube. Typical sample fluid transport tube diameters on the sample location side (upstream) of the filter are 0.25 inches or greater and inside diameters are 0.18 inches or greater to increase the time before such particles obstruct the fluid transport tube and reduce sample flow. If the fluid transport tube is easily plugged by particles, a conventional remedy is to increase the cross sectional diameter of the fluid transport tube and the filter. This increase allows more solid matter to accumulate before the sampling system must be cleaned. However, the volume increase this causes slows the transit time of the sample. To compensate for this delay, a higher capacity pumping system is usually added.
Use of conventional diameter sample fluid transport tubing also constrains a sampling system in two other ways. First, if gases or liquids are sampled from furnaces or other hot sources, they must be cooled prior to entry into the analyzer. The more rapidly the fluid is cooled, the less time is involved for reactions to take place that could change the composition of the fluid. For example, when carbon monoxide gas is extracted from an ambient pressure furnace at temperatures higher than approximately 1300 Deg. F., and then is cooled in or by the sampling system to below approximately 700 Deg. F., some of the carbon monoxide reacts with itself to form carbon dioxide and carbon (as soot). The slower the cooling takes place, the greater is the conversion. Because the unit fluid transport tubing wall surface area to internal cross-sectional volume of small diameter is greater than larger diameter fluid transport tubing, fluids in small diameter fluid transport tubes cool more rapidly than those in larger diameter fluid transport tubes. The result is a more accurate chemical analysis of the sample being extracted. Also, quicker cooling deposits less of a material (like soot) that is forms in the fluid transport tube during cooling. Second, assuming a constant fluid sample velocity, smaller diameter fluid transport tubing allows less gas to be removed for analysis. In processes that use or generate small gas volumes (such as research bioprocess reactors or vacuum furnaces, this feature is important.
Regardless of sample fluid transport tube diameter, in cases where large solids or particulates can plug the sample line or the filter in an unacceptably short time, additional components are often added to the sampling system. Most often, these components periodically reverse sample flow or reverse flow (“blow back”) a non-reactive usually inert clean fluid different from the sampled fluid in an attempt to remove accumulated particles from the unfiltered portion of the sample fluid transport tube and the filter itself. When a sample system that uses periodically reversing flow also includes multiple sample locations supplying a single analyzer, a second pump (“purge pump”) is sometimes used to draw fresh sample into fluid transport tubing and filters prior to analysis. This pump adds to the total sample flow through each line and can accelerate sample line plugging.
In many cases, more even more complex sampling systems are employed. When some or all of the fluid can: 1, change phase (if a gas, change into a liquid and if a liquid change into a solid); 2, react in the fluid transport tube to become viscous or form a solid; or 3, if the entrained solid particles are the constituent to be analyzed, the fluid transport tubing can be heated to prevent reactions or condensation or a fluid diluent can be added at or near the sample location. While both these approaches can function with certain fluids, their addition to the sampling system adds considerable complexity and expense to the system cost and they are employed only when necessary.
None of these known fluid sample systems and enhancements can properly handle some types of fluids that are useful to measure. If the fluid contains complex mixtures of chemicals and particles that interact, change phase, are withdrawn from high temperature or high pressure sample location or if complex chemical interactions occur in the sampling system, extractive sampling systems are often unreliable and ineffective. Some examples of such complex mixtures include: combustion gases, especially those produced by burning coal; industrial gas mixtures used for metal refining, processing and treating; certain chemical manufacturing and petroleum refining processes; gases and liquids from bioprocess fermentation and cell culturing; and almost all unfiltered air and wastewater emissions. Useful analytical information that can significantly improve manufacturing processes, reduce energy use and improve environmental compliance is lost when complex fluids affect the sampling system.
As an example, the complex composition of coal combustion gas is known to require several existing extractive sampling system enhancement combinations. Even with the enhancements, they are considered unreliable and are rarely used for continuous analysis.
FIG. 1 presents a diagram of a chemical analyzer with a typical current practice sampling system for measuring the chemical composition of coal combustion products. A typical combination includes: 1, heated sample fluid transport tubing UT1-UT8, each having a respective length, LUi, or gas dilution (not shown); 2, corresponding sample particle filters FS1-FS8, associated with each of the inlet sample fluid transport tubing lines, UT1-UT8, respectively, placed near the sample location to keep the majority of the sample line free from such particles; and 3, periodic gas blow back using clean and dried air or nitrogen to dislodge accumulated sample particles including Fluid 1 Supply 101, Filter FF1 106, and Purge Valves VP1. The outlet side of the sample particle filters FSi is coupled to the common gas analyzer 110 through a respective down stream fluid transport tubing DT1-DT8, each having a respective length, LDi, and a respective series of valves including the Purge Valves VPi, the Sample select valves VSi, and Sample Flow Regulation Valve VFS, 112
For process control, automatic and continuous analyzer operation using corresponding valves, VS1-VS8, to sequentially select several sample locations is typically desired (also shown in FIG. 1). For industrial applications such a system should operate at least six months without servicing or they are considered “unreliable”. However, using current practice sample system enhancements, sample system operation in a coal-fired power plant typically requires weekly service to prevent plugging. Attempts to improve sample system life by increasing the sample line cross-section and the sample flow rate may or may not extend sampling system operating time, however plugging still occurs in much less than 6 months and do to slower cooling, analysis samples from hot locations may become less accurate.
The reduced time before plugging in this and similar applications is caused by at least three properties of the sampled gas. First, the density of solid particles is very high and their size very small which allows them to be effectively entrained in the sample gas. These particles rapidly fill the portion of the sampling system fluid transport tubing LUi upstream of the filter and rapidly accumulate on the filter itself.
Second, coal combustion gas contains a significant portion of condensable chemicals that are in vapor phase in the combustion gas but partially condense even when sample systems are heated to high temperatures or dilution systems are used. The condensables tend to coat the inside diameter of the sample lines upstream of the filter (in combination with the particles present) and progressively decrease the remaining inside diameter reducing sample gas flow. Many of the condensables are very viscous and difficult to remove by blow back.
Third, the condensed liquids and the deposited particles inside the fluid transport tubes chemically react at the points of temperature transition and form a solid cement-like material that adheres to the inside sample fluid transport tube wall and filter. The combination of the condensables' viscosity and there reactions render gas blow back ineffective.